Isotopic Insights into the Origin of Dissolved Sulfate and the Sulfate-supported Microbial Processes in Deep Subsurface Fracture Waters in the Witwatersrand Basin, South Africa

Subsurface groundwater has been discovered from rock fractures in Precambrian cratons worldwide. In the Witwatersrand Basin (South Africa), the fracture waters show residence times in the order of 1 to 100 million years. Interestingly, sulfate-reducing bacteria are widespread in these fracture waters. In particular, a unique, single-species microbial ecosystem coupling sulfate reduction and H2 oxidation has been discovered in one of the oldest fracture waters. While H2 can be produced by water radiolysis induced by radioactive decay of U, Th and K in the host rocks, the origin of dissolved sulfate, the other essential energy source supporting the sulfate reducers, is poorly constrained. Here, we employ multiple sulfur isotopes (32, 33, 34, 36S) and oxygen isotopes (16, 18O) to assess potential sulfate sources as well as secondary processes that could affect the sulfur and oxygen isotope compositions of dissolved sulfate in the fracture waters. Twenty fracture water samples were collected from the Kloof, Tau Tona, Driefontein, Beatrix, Joel and Masimong gold mines intersecting three major sedimentary sequences, i.e., the Transvaal, Ventersdorp and Witwatersrand Supergroups, in the Witwatersrand Basin. Dissolved sulfate shows isotopic ranges from -5.3‰ to 19.4‰ for δ34S, -0.17‰ to 0.50‰ for Δ33S, and -1.1‰ to 10.9‰ for δ18O. The non-zero Δ33S values clearly indicate a sulfur source older than 2.0 Ga. Because sulfate minerals are lacking in the study area, and carbonate associated sulfate in the Transvaal dolomite overlying some of the water-bearing layers displays distinct δ34S (31.4-39.2‰) and Δ33S values (-0.01 to 0.16‰), dissolved sulfate could not have originated from dissolution of sulfate minerals in the host rocks. Instead, sulfide minerals in the Archean host rocks are more likely the source of dissolved sulfate. iii Two possible mechanisms can transform sulfide to sulfate: oxidative weathering of sulfide, which usually occurs in aerobic conditions, and/or radiolytic oxidation of sulfide, which can occur in both aerobic and anaerobic conditions. Comparison of the δ18O values between dissolved sulfate and their hosting fracture waters indicates that dissolved sulfate in relatively shallow samples (e.g., <1.9 km) fall into the expected δ18O range of the initial sulfate product derived from oxidative sulfide weathering but have not reached oxygen isotope equilibrium with hosting water. Because the periods required for oxygen isotope exchange to reach equilibrium are modeled to be less than 342,400 years, orders of magnitude shorter than the estimated residence times, the oxygen isotope disequilibrium implies that the sulfate production is fairly recent and probably still ongoing while the fracture waters have been isolated from the surface for millions of years. In contrast, the dissolved sulfate and their hosting waters from deep boreholes (>3 km) show apparent oxygen isotope equilibrium. This might be attributed to (1) accelerated oxygen isotope exchange rates at relatively high temperatures, (2) less efficient sulfate production mechanism (e.g., radiolytic oxidation), (3) larger extent of sulfate reduction, and/or (4) oxygen isotope shift by fluid-rock interaction. Overall, our results suggest that sulfate can be produced by in-situ oxidation of sulfide in the host rocks through oxidative sulfide weathering or radiolytic sulfide oxidation during water-rock interaction. Therefore, geological processes can provide a stable long-term sulfate source to sustain the terrestrial subsurface microbial systems isolated for geological time scales. In all sites except Beatrix, the large offset in δ34S values (up to 26.4‰) and compatible Δ33S values between dissolved sulfate and sulfide are consistent with the existence of sulfate-reducing bacteria in these waters. In Beatrix, however, sulfur isotopic signatures are more complicated. A series of samples collected from one borehole (depth of 1.3 km) in Beatrix between 2011 and 2013 show that dissolved sulfate and sulfide initially (in 2011) have very distinct Δ33S iv values, suggesting different sources for dissolved sulfate and sulfide. The Δ33S discrepancy between sulfate and sulfide diminished from 2011 to 2012 associated with an increase in δ34S value of sulfide and a decrease in δ34S value of sulfate, indicating bacterial sulfide oxidation was dominant over this time. In 2013, the δ34S and Δ33S signatures of sulfate and sulfide appear to be controlled by bacterial sulfate reduction. This 3-year sampling revealed a surprisingly quick ecological shift from one dominated by sulfide-oxidizing bacteria to one dominated by sulfate-reducing bacteria in the subsurface fracture waters, which has never been observed before.

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